Process for removing soap-contaminated conversion layers on metal workpieces

ABSTRACT

The process for removing a soap-contaminated conversion layer remaining on a metal workpiece after cold-forming with an aqueous acidic cleaning composition containing water, a surfactant such as an alkyl amine ethoxylate and nitric acid, phosphoric acid and/or amidosulphonic acid includes immersing the contaminated metal workpiece in the aqueous acidic cleaning composition at a temperature above 60° C. until it has a clean bright surface and a fatty acid formed by reaction of the soap in the conversion layer is dispersed in the cleaning composition and then subsequently separating the metal workpiece from the cleaning solution and, after the separating, cooling the recovered aqueous acidic cleaning composition to a temperature below 55° C. until a fatty acid layer including the fatty acid is separated from the aqueous acidic cleaning composition. The fatty acid layer is removed from the aqueous acidic cleaning composition to form a reusable aqueous acidic cleaning composition and then other metal workpieces are cleaned with the reusable aqueous acidic cleaning composition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for removing soap-contaminatedconversion layers remaining on metal workpieces after cold-forming bymeans of aqueous, acid cleaning agents as well as the use thereof forcleaning soap-contaminated calcium aluminate coatings remaining onworkpieces of aluminium or aluminium alloys after cold-forming.

In connection with the industrial manufacture of workpieces by means ofa non-cutting cold-forming operation, it is for instance necessary toremove remaining lubricant-contaminated phosphate, oxalate or aluminatecoatings, so-called soaked-in phosphate, oxalate or aluminate coatings.As lubricant, in particular oils and soaps are used, which chemicallyreact with the previously applied phosphate, oxalate or aluminatecoating, thereby forming an intimate bond. The preparation ofcold-forming by applying phosphate, oxalate and aluminate coatings andthe subsequent treatment with lubricants is commonly used in particularin the field of tube drawing, wire drawing, cold extrusion or sinking.

The above-mentioned soaked-in coatings must be removed above all when itis for instance intended to perform a soft annealing of the workpiece, afurther chemical surface treatment or a lacquer coating.

2. Prior Art

The acid cleaning of aluminium, in particular of cold-formed aluminiumis known and described for instance in U.S. Pat. No. 3,969,135 withrespect to the cleaning of aluminium cans provided with lubricantresidues, which cans were produced by deep-drawing or sinking from thinround sheet metal blanks. As a basis of the acid cleaning agent, thereis used sulfuric acid with a minor addition of hydrofluoric acid. Afurther acid cleaning agent for aluminium or aluminium alloys isdescribed in EP-A-275,043. As basis there is used a mineral acid fromthe group including sulfuric acid, phosphoric acid and nitric acid. Boththe aforementioned processes have in common that the aluminium oraluminium alloy surface was formed without preceding conversiontreatment by merely using lubricant.

The DE-A-3,843,148, however, describes a process for removing soaked-inconversion and soap layers with alkaline cleaning agents and under theinfluence of ultrasound. After a correspondingly high load, theconstituents of the detached conversion layers accumulate in thecleaning bath, and the bath must be disposed of, which is generallyeffected by means of a breakdown with acid. It is a disadvantage of thisprocess that major amounts of a strongly alkaline solution must not onlybe neutralized, but also greatly acidified. In the alkaline cleaningespecially of aluminium or aluminium alloys it is furthermoredisadvantageous that when using strongly alkaline cleaning agents thematerial is solubilized, or when using mild alkaline cleaning agents, anonly very slow and incomplete detachment of the lubricant-contaminatedconversion layer is effected even under the influence of ultrasound.

SUMMARY OF THE INVENTION

It is the object underlying the invention to provide a process forremoving soap-contaminated conversion layers remaining on metalworkpieces after cold-forming by means of aqueous cleaning agents, whichprocess eliminates the known, in particular the aforementioneddisadvantages and provides for a long dwell time of the cleaning agent.

This object is solved in that the process as described above isdeveloped such that the cleaning by means of a cleaning agent containingnitric acid, phosphoric acid and/or amido-sulphonic acid is performed ata temperature above 60° C., and the cleaning agent is cooled after asufficiently high load in the absence of the cleaned workpieces to atemperature below 55° C., the fatty acid layer floating up is separated,and at least the greater portion of the aqueous phase is reused forcleaning workpieces.

By means of the inventive process, the soaked-in, soap-contaminatedconversion layers are removed from the workpiece quickly and completely.The fatty acid formed from the soap remains dispersed in the cleaningagent at the employed cleaning temperatures of above 60° C., so thatanother deposition of fatty acid onto the workpieces is reliablyavoided.

After a sufficiently high load of the cleaning agent, the same is cooledto a temperature below 55° C. The fatty acid floats up and can beseparated most easily. Cooling and removal of fatty acid may be effectedin the cleaning bath itself, e.g. by means of stripping. This measureleads to the fact that the cleaning treatment must be interruptedtemporarily. For the case that this is not desired, part of the loadedcleaning agent can be withdrawn from the cleaning bath, be cooled in aseparate device, and be liberated from fatty acid floating up. Theseparation can likewise be effected by means of stripping, but in thecase of this procedure, the separation of the fatty acid by means offiltration or centrifugation should be preferred.

When removing the soaked-in conversion and soap layers, acid is consumeddue to a chemical reaction. In accordance with an advantageous aspect ofthe invention it is therefore provided to adjust the aqueous phase to bereused for cleaning to the nominal concentration of thecleaning-efficient components, in particular the acid. The amount ofacid to be added can easily be determined by means of an acid-basetitration.

In accordance with a further preferred embodiment of the invention,cleaning is effected with a cleaning agent containing a surfactant.Surfactants on the basis of alkylamine ethoxylates are particularlyadvantageous. Alkylamine ethoxylates react cation-actively in an acidmedium, and nonionogenically in a neutral medium. Examples for suchsurfactants are GENAMIN® C100 of Hoechst AG, described as ethoxylatedcoconut fatty amine containing 10 mol ethylene dioxide per molecule, orMARLAZIN® L10 of Chemische Werke Huls AG, described as fatty amineethoxylate.

In addition to a considerable extension of the dwell time of thecleaning agent by periodically separating the fatty acid floating up,the process in accordance with the invention in particular offers theadvantage that workpieces of aluminium or aluminium alloys can perfectlybe cleaned.

The advantages of the inventive process are particularly pronounced whenin accordance with a further aspect of the invention the process isapplied to the cleaning of soap-contaminated calcium aluminate coatingsremaining on work-pieces of aluminium or aluminium alloys aftercold-forming. In conjunction with soap, aluminate coatings arepreferably used as conversion layers on thick-walled aluminiumwork-pieces, see also G. Siemund "Schmieren und Phosphatieren beimKaltumformen kombinieren", Drahtwelt, issue no. 11, 1963.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be explained by way of example and in detail withreference to the following examples.

EXAMPLES

Shock absorber tubes are produced by cold extrusion from round aluminiumblanks, which prior to forming had been provided with a conversion layerof calcium aluminate and had subsequently been treated with a reactivesoap. Upon forming, the coating of calcium aluminate and soap was about10 g/m². The shock absorber tubes were cleaned in accordance with theinvention by means of immersion at 70° C. for a period of 10 minutes,subsequently rinsed with water and dried. In the process in accordancewith the invention the following cleaning agents were used:

    ______________________________________                                        Cleaning agent A:                                                                            5%    HNO.sub.3 (100%)                                                      0.2%    Genamin ® C100 (Hoechst AG)                                       rest    water                                                    Cleaning agent B:                                                                            2%    amidosulphonic acid (100%)                                            0.3%    (Genamin ® C100 (Hoechst AG)                                      rest    water                                                    ______________________________________                                    

By way of comparison, the subsequently described cleaning agents wereused:

    ______________________________________                                        Comparative cleaning agent 1:                                                                    80 g/l   sodium hydroxide                                                     20 g/l   sodium gluconate                                                      5 g/l   sodium dodecyl-                                                               benzenesulfonate                                                     rest     water                                             Comparative cleaning agent 2:                                                                    20 g/l   H.sub.2 SO.sub.4                                                     rest     water                                             Comparative cleaning agent 3:                                                                    20 g/l   HCl                                                                   3 g/l   Genamin ® C100                                                            (Hoechst AG)                                                         rest     water                                             ______________________________________                                    

The cleaning experiments performed with the above-mentioned cleaningagents led to the following result: Comparative cleaning agent 1 turnedout to be completely useless. In addition to a considerable generationof hydrogen, the surfaces of the workpieces were rough, stained and dullupon rinsing.

Comparative cleaning agent 2 initially led to clean, bright surfaces.But after the throughput of a few workpieces, a crystalline coatingremained on the surface of the shock absorber tubes upon cleaning thesame, which coating was also maintained upon adding cleaning-efficientcomponents and increasing the acid concentration. On the surface of thecleaning bath, there was additionally deposited a greasy film, whichsubstantially consisted of fatty acid and persistently adhered to thetreated parts. Cleaning with comparative cleaning agent 3 also producedclean surfaces first of all. But there was observed some pitting easilyrecognizable with the naked eye.

On the other hand, the results achieved with cleaning agents A and Bwere satisfactory in every respect. There were not only achieved clean,bright surfaces, but the cleaning effect remained perfect even after anextended throughput of the workpieces. The cleaning agents turned milkyduring their usage, but upon rinsing the workpieces could stillcompletely be wetted with water.

In order to maintain the cleaning effect of cleaning agents A and B, abath sample of 5 ml was withdrawn at certain intervals and titrated with0.1 n sodium hydroxide solution against bromocresol green as indicator.The consumption of 1 ml 0.1 n sodium hydroxide solution corresponds toabout 2 g/l free amidosulphonic acid or 1.3 g/l nitric acid.Corresponding to the titration result, the cleaning agents A and B werecompleted with amidosulphonic acid or nitric acid as well as with thesurfactant GENAMIN® C100 (ethoxylated coconut fatty amine containing 10mol ethylene dioxide per molecule) in the same proportion as whenpreparing the cleaning agent.

After a throughput of 2500 m² workpiece surface per m³ cleaning agent,the temperature of the cleaning bath was decreased to 45° C. Thedispersed fatty acid solidified, floated up in granular, compact form,and was skimmed off.

After another completion of the cleaning-efficient components to theinitial concentration, the cleaning agent was again fully usable.

In a further experiment, phosphatized, soap-contaminated andsubsequently pressed steel screws were treated at 65° C. in a rotatingdrum for 10 minutes in a cleaning agent of the following composition,subsequently rinsed with water and dried.

The cleaning agent contained

150 g/l phosphoric acid

1 g/l FERHIBIT® S, a pickling inhibitor of CHEMETALL GmbH

2 g/l MARLAZIN® L10 (fatty amine ethoxylate)

rest water

Due to the aforementioned treatment, the phosphate and soap coating wascompletely removed. The concentration of cleaning-efficient componentsin the cleaning agent was monitored by titration, consumed acid as wellas other constituents of the cleaning agent were completed as describedabove. The regeneration of the loaded cleaning agent was effected bycooling to 40° C., removing the granular fatty acid floating up, andreadjusting the cleaning-efficient components to the originalconcentration. The cleaning agent could be maintained efficient forseveral weeks.

What is claimed is:
 1. A process for removing a soap-contaminatedconversion layer remaining on a metal workpiece after cold-forming themetal workpiece, wherein said soap-contaminated conversion layercomprises a soap and at least one member selected from the groupconsisting of phosphates, oxalates and aluminates, said processcomprising the steps of:a) providing an aqueous acidic cleaningcomposition comprising water and at least one acid ingredient selectedfrom the group consisting of nitric acid, phosphoric acid andamidosulphonic acid; b) immersing the metal workpiece with thesoap-contaminated conversion layer in the aqueous acidic cleaningcomposition at a temperature above 60° C. until the soap-contaminatedconversion layer is removed from the metal workpiece and until a fattyacid formed from the soap in the conversion layer is dispersed in theaqueous acidic cleaning composition; c) after step b), separating themetal workpiece from the aqueous acidic cleaning composition; d) afterstep c), cooling the aqueous acidic cleaning composition to atemperature below 55° C. until a fatty acid layer comprising the fattyacid is separated from said aqueous acidic cleaning composition; e)removing said fatty acid layer from said aqueous acidic cleaningcomposition to form a reusable aqueous acidic cleaning composition; andf) cleaning other metal workpieces with at least a portion of thereusable aqueous acidic cleaning composition.
 2. The process as definedin claim 1, wherein said metal workpiece is an aluminum workpiece or analuminum alloy workpiece and the soap-contaminated conversion layerconsists of soap-contaminated calcium aluminate.
 3. The process asdefined in claim 1, wherein said aqueous acidic cleaning compositionincludes a surfactant.
 4. The process as defined in claim 1, furthercomprising adding a predetermined concentration of said at least oneacid ingredient in said aqueous acidic cleaning composition.
 5. Theprocess as defined in claim 3, wherein said surfactant is alkylamineethoxylate.